Loaders for loading soil on carrier vehicles operates at construction sites. Wheel loaders are known as one type of loaders. A wheel loader includes a work machine having a boom and a bucket, unload soil shoveled with the bucket into a vessel of a dump truck, which is one type of carrier vehicles. The operator of the wheel loader carries out unloading operation of unloading soil on the bucket into a vessel by manipulating a control lever to adjust the position of the boom and the angle of the bucket. Patent Literature 1 discloses a technology for controlling a boom cylinder and a bucket cylinder so as to prevent soil from dropping out of a bucket.